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Missing fossil link ’Dallasaurus’ found

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When amateur fossil finder Van Turner discovered a small vertebra at a construction site near Dallas 16 years ago, he knew the creature was unlike anything in the fossil record. Scientists now know the significance of Turner’s fossil as the origin of an extinct line of lizards with an evolutionary twist: a land-dwelling species that became fully aquatic.

Turner took the remains to paleontologists at the Dallas Museum of Natural History, but it took several years before scientists dubbed the find Dallasaurus turneri. Word of Dallasaurus is now reaching the scientific community with a special issue of the Netherlands Journal of Geosciences, featuring an article by Southern Methodist University paleontologist Michael Polcyn and Gordon Bell Jr. of Guadalupe National Park in Texas.

They describe Dallasaurus, a three-foot long lizard who lived 92 million years ago in the shallow seas and shores of what was then a stretch of Texas mostly under water, and also used the fossil to better understand the mosasaur family tree. Polcyn and Bell painstakingly pieced together an understanding of the anatomy and natural history of Dallasaurus from the bones Turner discovered and from some matching skeletal remains at the Texas Memorial Museum at the University of Texas in Austin.

Dallasaurus represents a missing link in the evolution of a group of creatures called mosasaurs, prehistoric animals that started out on land, but evolved in the seas and dominated the oceans at the same time dinosaurs ruled the land. One aspect of Polcyn and Bell’s research is the revelation that Dallasaurus retained complete limbs, hands and feet suitable for walking on land, whereas later mosasaurs evolved their limbs into flippers.

“This is pretty close to the beginning of the mosasaur family tree,” says Dallas Museum of Natural History Earth Sciences Curator and SMU Adjunct Professor of Paleontology Anthony R. Fiorillo, Ph.D. “It is the most complete mosasaur retaining all of its limbs found in North America.”

Mosasaurs, every bit as prolific, fascinating and nearly as big as some dinosaurs, are becoming more popular for paleontologists to study. Mosasaurs lived and became extinct alongside dinosaurs, but few paleontologists specialize in them. Later mosasaurs grew as large as their dinosaur brethren, reaching up to 45 feet in length. Until the discovery of Dallasaurus, however, only five primitive forms with land-capable limbs were known, all of them found in the Middle East and the eastern Adriatic.

“Lizards had nearly 150 million-year-long history on land; then in the Late Cretaceous, the final stage of the age of dinosaurs, one group moved into the sea and rose to the very top of the food chain,” explains Polcyn, director of SMU’s Visualization Laboratory, part of the university’s geological sciences department. “Starting out as small animals like Dallasaurus, they mastered their new marine environment and rose to become the top predator in their ecosystem, the T. Rex of the ocean.”

The Late Cretaceous period was a time of hot house temperatures and rising sea levels.

“As the earth warmed and the seas rose, small land-dwelling lizards took to the oceans and developed increasing levels of seagoing capabilities, and over 30 million years, eventually evolving into the top predator of their domain before becoming extinct some 65 million years ago” says Polcyn.

The advanced fin-bearing mosasaurs have been grouped into three major lineages. Although a small number of primitive mosasaur have been known to retain land-capable limbs, they were thought to be an ancestral group separate from the later fin-bearing forms. Dallasaurus represents a clear link to one lineage of the later forms and the first time researchers can clearly show mosasaurs evolved fins from limbs within the different lineages of mosasaurs.

With the aid of computer science and SMU’s visualization laboratory, Polcyn has been able to simulate what Dallasaurus looked like, and how, based on his skeletal remains, he would swim and move from land to sea. An artist has taken Polcyn’s visualization work even one step further by creating a life-sized model of Dallasaurus, a work that is soon to be on display at the Museum along with the computer simulation.

When funds become available for reconstructing a suitable exhibit, the bones of Dallasaurus will be displayed at the Dallas Museum of Natural History. The work, however, will take several more years of additional efforts and substantial funding. A nearly 30-foot long mosasaur, some 75 million years old, already is on display at the Dallas Museum.

Major dinosaur finds are frequently the result of creatures dying in groups through flooding or drought, situations that lend themselves fairly well to more complete preservation and conservation of their bones, and much slower deterioration. Mosasaur fossils, in contrast, are rarely found in large groupings, and are only found in areas once covered by seas. Remains were quick to deteriorate under ocean currents; their bodies often fell victim to the ravages of other sea life, such as sharks, who would pick away at carcasses for food. Because of their mostly shallow sea and seaside habitats, the remains of early mosasaurs are even more rare and much harder to find.

But in the last two decades, many new discoveries and significant advances have been made in the understanding of mosasaur evolution and how they lived. Dallasaurus significantly advances that understanding by filling in a long missing piece of mosasaur evolution, specifically a time at which they transition from land to sea.

The importance of Turner’s discovery isn’t lost on the researchers putting together the pieces of the mosasaur puzzle. In fact, they predict the legacy of Turner’s discovery will live on. His contribution was honored by naming the species, “turneri,” after his last name. “Not all major discoveries are made by highly trained paleontologists,” notes Dallas Natural History Museum Curator Fiorillo. “The observant individual, even kids, can still make an important find,” he says. “Once this goes mainstream, and people begin to recognize what mosasaurs are, we’ll be finding more and more.”

Meredith Dickenson | Quelle: EurekAlert!
Weitere Informationen: www.dallasdino.org
www.smu.edu

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